Electrical Equipment Room Design Considerations

8/8/2019 Electrical Equipment Room Design Considerations

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Electrical Equipment Room Design

Considerations

Atlanta Chapter IEEE Industry

Applications Society

presented at the

Sheraton Buckhead Hotel

Atlanta, Georgia

November 20, 2006


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Outline

1. Definitions2. Power Distribution Configurations

3. Selection of Transformer

4. Installation and Location of Transformer

5. Service Entrance Equipment

6. Selection of Circuit Breaker

7. Electrical Equipment Room Construction (New)

8. Electrical Equipment Room Construction

(Existing)

9. Maximum Impedance in a Ground Return Loop to

Operate an Overcurrent Protective Device

10. 2005 NEC Requirements


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Outline

11. Ground Fault Sensing12. Zero Sequence Sensing vs. Residual Sequence

Sensing

13. Power Distribution Systems with Multiple Sources14. Modified Differential Ground Fault (MDGF)

Protection Systems

15. Designing a MDGF Protection System

16. Reported Ground Fault Losses


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1. Definitions

The system configuration of any PowerDistribution System is based strictly on howthe secondary windings of the Power Class

Transformer, or generator, supplying theService Entrance Main or loads, areconfigured. (This includes whether or not

the windings are referenced to earth.) The system configuration is not based on

how any specific load or equipment is

connected to a particular power distributionsystem.

System Configuration


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1. Definitions

A designed, coordinated, functional, and properly installedsystem that provides

protection from electrical faults or short

circuit conditions that result from any

unintentional, electrically conducting

connection between an ungroundedconductor of an electrical circuit and the

normally noncurrent-carrying conductors,

metallic enclosures, metallic raceways,metallic equipment, or earth.

Ground Fault Protection System


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1. Definitions

A system intended to provide protection ofequipment from damaging line-to-ground

fault currents by operating to cause a

disconnecting means to open all

ungrounded conductors of the faulted

circuit. This protection is provided at currentlevels less than those required to protect

conductors from damage through the

operation of a supply circuit overcurrentdevice.

Ground Fault Protection of Equipment

(Per Article 100 in the 2005 NEC)


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1. Definitions

An unintentional, electrically conducting

connection between an ungroundedconductor of an electrical circuit and the

normally noncurrent-carrying conductors,

metallic enclosures, metallic raceways,metallic equipment, or earth.

Ground Fault

(per Article 250-2 in the 2005 NEC)


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1. Definitions

An electrically conductive path from

the point of a ground fault on a wiringsystem through normally noncurrent-

carrying conductors, equipment, or the

earth to the electrical supply source.

Ground Fault Current Path

(per Article 250-2 in the 2005 NEC)


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2. Power Distribution Configurations


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3. The Selection of the Power Class

Transformer to Install

Dry Type Air/Dry

Cast Resin

Liquid Filled Mineral Oil (flammable, marketed or perceived biohazard)

Silicone (non-flammable)

Bio [Corn Oil] (flammable, but not a marketed orperceived bio hazard )

Gas Type


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4. The Location and Installation of the

Power Class Transformer

Outdoors Enclosure Types Relative to Environment

Accessibility

Camouflage to Meet Local Requirements Hazardous Locations

Protection for and from people

Required Markings

Security

Indoors To Vault or Not To Vault

Fire Ratings of Location

Rating of NEMA Environment

Environmental Considerations

Cooling Ability

Accessibility

Security


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Switchboards (UL 891)

The UL 891 Standard applies to switchboardsnominally rated at 600 V or less and intended for

use in accordance with the Canadian ElectricalCode, Part 1 (CE Code, Part 1), the NationalElectrical Code (NEC), ANSI/NFPA 70, and theMexican Standard for Electrical Installations(Utility), NOM-001-SEDE.

The term switchboard is intended to refer to adead-front switchboards only.

Not associated with the following switchboardtypes:

Preset and dimmer control (theater) Live front, railway control, or electrification

Constructions intended only for receiving motorcontrol center units.

Covers switchboards for use on circuits havingavailable rms symmetrical short-circuit currents ofnot more than 200 000 A.


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Switchboards (UL 891) continued

Traditionally have less bus clearances thanswitchgear.

Bussing is rarely insulated (unless specified andpaid for).

The spaces between all distribution circuitbreakers are not usually compartmentalized.

Employs a greater use of barriers within a sectionversus compartments.

There are barriers between sections.

Circuit Breakers employed within a switchboardsection can be molded case, insulated case, or apower type.

Instantaneous trip functions on molded case andinsulated case circuit breakers employed within aswitchboard assembly are not generally ANSI

rated for higher short time withstand capability Can possibly be accessible from only one side.


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Switchgear (UL 1558)

Covers metal-enclosed low-voltage power circuitbreaker switchgear assemblies containing but not

limited to such devices as low-voltage powercircuit breakers, other interrupting devices,switches, control, instrumentation and metering,protective and regulating equipment.

Covers power equipment intended for use inordinary locations in accordance with the NationalElectrical Code.

UL 1558 requirements are intended to supplementand be used in conjunction with the Standard for

Metal-Enclosed Low Voltage Power CircuitBreaker Switchgear, ANSI C37.20.1, and theStandard for Conformance Testing of Metal-Enclosed Low-Voltage AC Power Circuit BreakerSwitchgear Assemblies, ANSI C37.51.

Covers equipment rated 600 Vac or less nominal,635 Vac maximum.


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Switchgear (UL 1558) continued

For use on circuits having available rmssymmetrical short-circuit currents of not more than

200 000 A. Switchgear is 30 cycle rated and can be installed

without instantaneous overcurrent protection toassist in selectivity.

Traditionally switchgear has more bus clearances Insulated bussing is commonly employed as astandard

Compartmentalized cubicles versus barriersbetween sections

Circuit breakers employed are by definition (ANSIstandards) classified as power class circuitbreakers having a higher short time withstandcapability than switchboards.

Switchgear is front and rear accessible.


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6. Selection of MCCB Circuit Breakers as

Overcurrent Protective Devices

Molded Case Circuit Breakers (MCCB)

Thermal Magnetic

Low Voltage Applications up to 600Vac

Typical Current Ratings are from 5A to 2000A

Originally designed for overload and phase-to-phase shortcircuit protection

If intended to provide ground fault protection, these typecircuit breakers must be employed on a circuit with aneffective low impedance, ground fault current return path

Mechanism is totally sealed Mainly require manual operations

Not designed or intended to be serviceable

Typically bolted onto or plugged into bus

Should be installed with equipment turned off

A ground fault module can be installed on some devices Most are only 80% rated. 100% rating is optional

Same size and ampere rating of same circuit breakers aremanufactured with various short circuit ratings. (Knowing theavailable short circuit current available at point ofconnections is imperative!)

MICROLOGIC or Electronic Trip Units Offers a combination of LSIG function to the specifier or user

Provides for improved time/current or fault coordinationfunction


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6. Selection of ICCB Circuit Breakers as


Insulated Case Circuit Breakers (ICCB)

Generally are only UL Rated and not ANSI Rated

Low Voltage Applications up to 600Vac Typical Current Ratings are from 400A to 4000A

They provide limited internal access to replacement of specific parts

Can be fixed mounted directly onto bus or manufactured as a draw-outtype within a specifically designed cubicle

Can be fitted at the factory with multiple auxiliary functions

Multiple Auxiliary Contacts Shunt Trip and Shunt Close Coils

LSIG Trip Alarms

Electric Motor Operator

Most are only 80% rated, 100% rating is optional

Ampere rating of circuit breakers can be manufactured with variousshort circuit ratings. (Knowing the available short circuit currentavailable at point of connections is imperative!)

MICROLOGIC or Electronic Trip Units

Offers a combination of LSIG functions to the specifier or user

Provides for improved time/current or fault coordination function


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6. Selection of PCB Circuit Breakers as


Power Circuit Breakers (PCB) ANSI Rated and not UL Rated

Low Voltage Applications up to 600Vac

Typical Current Ratings are from 800A to 6000A They provide internal access to inspection, maintenance, and

replacement specific parts

Manufactured as draw-out type construction with isolated compartmentor cubicle

Can be fitted at the factory with multiple auxiliary functions

Multiple Auxiliary Contacts Shunt Trip and Shunt Close Coils

LSIG Trip Alarms

Electric Motor Operator

100% rated

Tested and rated to a higher level of initial or asymmetrical short circuit

current More robustly constructed than ICCBs

Intended for longer use life and more open and close operations

MICROLOGIC or Electronic Trip Units with LSIG functions are standard

Has higher initial cost.


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7 Electrical Room Construction


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7. Electrical Room Construction(New) - continued

Conflicts between the architecture and electrical engineer of

record concerning available/needed space for all electrical

equipment rooms.

Dual use spaces (Electrical Equipment Room and StorageArea)

Fire alarms

Security Cameras and their location

Is all electrical equipment effectively bonded and grounded?

Effective lighting within electrical rooms or closets.

Equipment installation requirements and guidelines provided by

the electrical equipment manufacturer

Operations and Maintenance Manuals provided by the

electrical equipment manufacturer

Startup and Testing requirements

Lock Out / Tag Out Requirements

Danger and Warning Labels (Inform and Warn Requirements)

ec r ca oom ons ruc on


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. ec r ca oom ons ruc on(New) - continued

The need for a house keeping pad under the equipment.

Locations within a specific building or facilities for the

placement of service equipment, distribution equipment, power

panels, motor control centers, ATS Units, battery banks, UPSUnits, onsite generators, etc

Requirements for any house keeping pad constructed (material

composition and necessary height)

Flooding concerns from either external and internal sources Structural and seismic requirements for the electrical room

Structural and bracing requirements for the electrical

equipment enclosures

Structural and bracing requirements for overhead busways,busducts, and conduits

Need for floor drains or sump pumps in electrical rooms

Critter protection

Internal heater and humidity controls

Will any local employees have access to water hoses???


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8 Electrical Room Construction


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8. Electrical Room Construction(New to Existing Equipment Additional

Considerations)

Bringing existing Service Equipment into

compliance to current electrical Codes

Additions of multiple power sources fromanother transformer or generator

Dissimilar power or service equipment

are rarely compatible (BIG RED FLAGS)

Are all power sources effectively bonded

together via a low impedance path Complex technical issues to resolve

because of incompatible ground fault

protection systems


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0.924 OHMS200A100A

1.23 OHMS150A75A

1.75 OHMS100A50A

2.31 OHMS80A40A

3.08 OHMS60A30A

4.62 OHMS40A20A

6.12 OHMS30A15A

9.23 OHMS20A10A

18.48 OHMS10A5A

* (480/1.732)/ 3A (or)

1.5b

Maximum %Z in Ground

Return Loop*

Circuit Breaker Ampere

Rating

Value b

Fuse Size in Amperes

Value a


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0.154 OHMS1200A600A

0.185 OHMS1000A500A

0.231 OHMS800A400A

0.308 OHMS600A300A

0.370 OHMS500A250A

0.462 OHMS400A200A

0.528 OHMS350A175A

0.616 OHMS300A150A

0.739 OHMS250A125A

* (480/1.732)/ 3A (or)

1.5b


Return Loop*


Rating

Value b


Value a


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0.047 OHMS4000A2000A

0.062 OHMS3000A1500A

0.074 OHMS2500A

0.093 OHMS2000A1000A

0.116 OHMS1600A800A

* (480/1.732)/ 3A (or)

1.5b


Return Loop*


Rating

Value b


Value a


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Ground-fault protection of equipment shall beprovided for solidly grounded wye electricalservices of more than 150 volts to ground butnot exceeding 600 volts phase-to-phase foreach service disconnect rated 1000 amperesor more. The grounded conductor for the

solidly grounded wye system shall beconnected directly to ground without insertingany resistor or impedance device.

The rating of the service disconnect shall beconsidered to be the rating of the largest fusethat can be installed or the highest continuouscurrent trip setting for which the actualovercurrent device installed in a circuit breakeris rated or can be adjusted.

Ground-Fault Protection of Equipment(per Article 230-95 in the 2005 NEC)


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The ground-fault protection systemshall operate to cause the servicedisconnect to open all ungroundedconductors of the faulted circuit. Themaximum setting of the ground-faultprotection shall be 1200 amperes, andthe maximum time delay shall be one

second for ground-fault currents equal toor greater than 3000 amperes.

Setting for Ground-Fault Protection(per Article 230-95(A) in the 2005 NEC)


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The maximum setting for ground-fault sensors is 1200

amperes. There is no minimum, but it should be notedthat settings at low levels increase the likelihood ofunwanted shutdowns. The requirements of 230.95 placea restriction on fault currents greater than 3000 amperesand limit the duration of the fault to not more than 1

second. This restriction minimizes the amount of damagedone by an arcing fault, which is directly proportional tothe time the arcing fault is allowed to burn.

Care should be taken to ensure that interconnectingmultiple supply systems does not negate proper sensingby the ground-fault protection equipment. A carefulengineering study must be made to ensure that faultcurrents do not take parallel paths to the supply system,thereby bypassing the ground-fault detection device. See215.10, 240.13, 517.17, and 705.32 for furtherinformation on ground-fault protection of equipment.

Ground-Fault Protection of Equipment(NECH Commentary for Article 230-95(A) in the 2005 NEC)


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The ground-fault protection system shall beperformance tested when first installed on site. The test

shall be conducted in accordance with instructions thatshall be provided with the equipment. A written record ofthis test shall be made and shall be available to theauthority having jurisdiction.

(The requirement for ground-fault protection systemperformance testing is a result of numerous reports ofground-fault protection systems that were improperlywired and could not or did not perform the function for

which they were intended. This Code and qualifiedtesting laboratories require a set of performance testinginstructions to be supplied with the equipment.Evaluation and listing of the instructions fall under the jurisdiction of those best qualified to make such

judgments, the qualified electrical testing laboratory (see90.7). If listed equipment is not installed in accordancewith the instructions provided, the installation does notcomply with 110.3(B).)

Testing the Ground-Fault ProtectionSystem (per Article 230-95(C) in the 2005 NEC)


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Electrical equipment and wiring and otherelectrically conductive material likely to becomeenergized shall be installed in a manner thatcreates a permanent, low-impedance circuit

facilitating the operation of the overcurrentdevice or ground detector for high-impedancegrounded systems. It shall be capable of safelycarrying the maximum ground-fault currentlikely to be imposed on it from any point on thewiring system where a ground fault may occurto the electrical supply source. The earth shall

not be considered as an effective ground-faultcurrent path.

Effective Ground-Fault Current Path(per Article 250-4(A)(5) in the 2005 NEC)

11 G d F lt S i


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11. Ground Fault Sensing

There are three basic types of sensor arrangement usedto sense abnormal current flow in a Zero Sequence

Relaying Scheme. They are;

A. Zero Sequence Sensing Where one sensor isemployed to surround all phase conductors and neutral

conductor.

B. Modified Zero Sequence Sensing Where one sensor is

employed to surround all phase conductors and a

separate sensor surrounding the neutral sensor.

C. Residual Sequence Sensing Where one sensor

surrounds each phase conductor and neutral conductor.

12 Zero Sequence Sensing vs Residual


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12. Zero Sequence Sensing vs. Residual

Sequence Sensing

Zero Sequence Sensing

Current sensors or current transformer

are external to disconnecting device

Usually associated with a separate and

externally mounted Ground Fault Relay No internal relationship to disconnect to

be operated. All GFP components are

externally mounted. Disconnect requires a shunt trip coil

Requires a separate and reliable sourceof control power. (Weakest Link)

12 Zero Sequence Sensing vs Residual


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12. Zero Sequence Sensing vs. Residual

Sequence Sensing

Residual Sequence Sensing

Phase sensors are usually internal to

protective device to be operated. Ground fault relaying is usually

performed internal to protective device

or circuit breaker to be operated.

No external control power supply

required. Protective device or circuit

breaker is self-powered.

Self powered residual sequence sensing

GFP is referred to as Integral GroundFault Protection.

13 Power Distribution Systems with


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13. Power Distribution Systems with

Multiple Sources

Two or more sources of power connected to the

switchboard or switchgear.

Separate Main Circuits Breakers

Tie Circuit Breakers

Separate Load Busses

Neutral Bus associated with each power source are

interconnected together. Multiple neutral-to-ground bonds via the connection of

multiple System Bond Jumpers and Main Bonding

Jumpers (MBJ).

The interconnection of multiple grounding conductors onthe line side of the main circuit breakers can function as

a parallel neutral bus.

Multiple current paths for imbalanced neutral currents

and ground fault currents to flow.

13 Power Distribution Systems with


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13. Power Distribution Systems with

Multiple Sources

14 Modified Differential Ground Fault


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14. Modified Differential Ground Fault

(MDGF) Protection Systems

The purpose of a MDGF Protection System is to senseand properly relay all of the phase and neutral currentswhich circulate through and within a multiple source

power distribution system at all times during normal andabnormal conditions.

The MDGF Protection System prevents nuisance trippingof circuit breakers from the flow of imbalanced currents

from imbalanced loads. The MDGF Protection System prevents nuisance tripping

of circuit breakers from the flow circulating currents inand out of the switchboard or switchgear from closed

transition operations. The MDGF Protection System opens all of the

appropriate circuit breakers closes to the faulted locationwithin a switchboard or switchgear.

The MDGF Protection System provides for thesectionalization of load busses.


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Before a MDGF Protection Systems for a

multiple sourced switchboard can be

configured the following must be evaluated.A. Type of circuit breaker?

1. The internal sensors and ground fault

relaying are not always configured the same.B. Frame size of the circuit breaker?

1. Different frame size breakers often have

different C.T. ratios.

2. Different series circuit breakers can have the

frame size and C.T. ratios, but different

instantaneous C.T. polarities.


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C. How are the circuit breakers supplied withpower?

1. Are the circuit breakers top feed?2. Are the circuit breakers bottom feed?

D. Are the circuit breakers fixed or draw outtype?

The MDGF must be maintained when anycircuit breaker is open or removed from its cell(drawn out).

E. Is the tie circuit breaker (52T) specified toground fault protection?1. Non-Relayed Tie Circuit Breaker has noGF Protection

2. Relayed Tie Circuit Breaker has GFProtection


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16. Reported Ground Fault Losses

One Major Loss (Claim) Every Two

Weeks

Special Notes for Consideration


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Special Notes for Consideration

Phase-to-ground faults are the most common form of electrical

faults. (95% to 98%)

During any phase-to-ground fault the ground fault return path is50% of the power circuit.

Phase-to-ground faults are the most destructive type of electricalfaults.

Contrary to popular belief or some marketing publications, fuses do

NOT provide selective coordination from most phase-to-groundfaults.

For a current limiting fuse to limit current as designed and intendeda fault must be a bolted fault. Current limiting fuses do not

coordinate well during high impedance phase-to-ground faults.

Multiple levels of ground fault protection via MICROLOGIC CircuitBreakers provides the best form of selective coordination fromphase-to-ground fault condition.

Complex ground fault protection systems (MDGF) require aspecific level of electrical engineering expertise.


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Code NEC, NFPA 70

The NEC is a minimum construction andinstallation requirement document.

The NEC is NOT a design or performancestandard .

Minimum requirements are often insufficient forthe construction and installation of mission criticalfacilities as Data Centers, Communication

Centers, Hospitals, Industrial Plants, MilitaryFacilities, Prisons, and Transportation Centers.


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Questions & Answers

Speaker Contact Information


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Speaker Contact Information

S. Frank Waterer III Staff EngineerSchneider Electric Engineering Services

2979 Pacific Drive, Suite ENorcross, Georgia 30071

Office Phone: 770-447-0351, Ext. 102

Office Fax: 770-447-4985

E-MAIL: [email protected]

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